1. Field of the Invention
The present invention relates to a wave power generation device, which generates power by extracting energy from waves through movement of a float floating on the sea and a method of controlling the same.
2. Description of the Related Art
Conventionally, there has been a wave bower generation device with a float floating on the sea or in the sea (see Patent Literature 1, for example). This wave power generation device has a power generator which generates power by converting an external force received by the float from waves to electricity. FIG. 4 illustrates an example of the wave power generation device. This power generating device 1X has a body 3, a float 4 which moves vertically along the body 3, a transmission mechanism 8 which converts the vertical movement of the float 4 into a rotary movement, and a power generator 5 connected to the transmission mechanism 8. This transmission mechanism 3 is constituted by a combination of a ball screw 6 and a ball nut 7, for example. The body 3 is installed as a floating type which is fixed to a sea floor by a mooring wire 11 or a bottom-mounted type which is installed upright from the sea floor.
Next, descriptions are given of an operation of the wave power generation device 1X. First, the float 4 of the wave power generation device 1X receives the external force from the waves and moves vertically. Along with the vertical movement of the float 4, the ball nut 7 fixed to the float 4 moves vertically. By means of this vertical movement of the ball nut 7, the ball screw 6 rotates and transmits the rotary movement to the connected power generator 5. Here, the left side of FIG. 4 illustrates a state in which the ball screw 6 rotates in an arrow direction by a rise of the ball nut 7, while the right side in FIG. 4 illustrates a state in which the ball screw 6 rotates in a direction opposite to the above by lowering of the ball nut 7.
With this constitution, the wave power generation device 1X can rotate the power generator 5 by a kinetic energy of the waves and generate power. Here, it is known that power generation efficiency can be improved by making the vertical movement of the float 4 resonate with a wave period (also called synchronization) and by giving an appropriate power generation load. Thus, regarding the float 4, its natural period is designed so that resonance (synchronization) occurs at a wave period of a sea area where the wave power generation device 1X is installed. Here, in order to make the vertical movement of the float 4 resonate (synchronize) at the wave period, it is required to make an inertia force and a restoring force of the float. 4 in the wave period equal to each other.
Descriptions are given below of a process in which a natural period T of the float 4 is determined. First, in general, a natural period To (sec) of a simple oscillating system having one end of a spring having a spring constant ko (N/m) fixed and the other end attached with a mass Mo (kg) can be expressed by the following formula:
                              T          0                =                  2          ⁢          π          ⁢                                                    M                0                                            k                0                                                                        [                  Formula          ⁢                                          ⁢          1                ]            
Subsequently, if the float 4 is floating on a water surface, a restoring force applied to the float (buoyancy less weight) works as a spring force. Assuming that the float 4 has a columnar shape, this spring constant k can be expressed by the following formula using a columnar sectional area S:k=ρgS  [Formula 2]
Here, reference character ρ denotes a water density (1000 kg/m3) and reference character g denotes gravitational acceleration (0.6 m/sec2). Moreover, a mass M (kg) of the float 4 can be expressed by the following formula, assuming that a draft (submerged depth) is df (m):M=ρSdf  [Formula 3]
As described above, the natural period T (sec) of the float 4 can be expressed by the following formula. Since the float 4 is on the water, an added mass with a mass ratio α is considered. This added mass refers to a mass apparently increased by movement of the peripheral water together with the vertical movement of the float 4:
                                                        T              =                            ⁢                              2                ⁢                π                ⁢                                                                                                    (                                                  1                          +                          α                                                )                                            ⁢                      M                                        k                                                                                                                          =                            ⁢                              2                ⁢                π                ⁢                                                                                                    (                                                  1                          +                          α                                                )                                            ⁢                      ρ                      ⁢                                                                                          ⁢                                              Sd                        f                                                                                    ρ                      ⁢                                                                                          ⁢                      g                      ⁢                                                                                          ⁢                      S                                                                                                                                              =                            ⁢                              2                ⁢                π                ⁢                                                                                                    (                                                  1                          +                          α                                                )                                            ⁢                                              d                        f                                                              g                                                                                                          [                  Formula          ⁢                                          ⁢          4                ]            
From the aforementioned formulas, it is known that the natural period T of the float 4 having a constant sectional area like a column has nothing to do with a diameter of the float 4 but depends only on its draft df. That is, when the wave power generation device 1X is designed, the natural period T of the float 4 is fixed to a period of a power generation design wave (wave desired to have maximum power generation efficiency) of the sea area for installation. On the other hand, the wave period Ts changes all the time in 4 to 10 seconds, for example.
Subsequently, descriptions are given of a principle by which the wave power generation device 1X absorbs wave energy and generates power. First, the vertical movement of the float 4 of the wave power generation device 1X on which only the power generator 5 is loaded can be expressed by the following formula:(M+m)z″+(N+d)z′+Cz=Fz  [Formula 5]
Here, reference character M denotes a mass of the float, m denotes the added mass, N denotes wave attenuation of the float, d denotes a load attenuation of the power generator, C denotes a restoring force coefficient of the float, and Fz denotes a compulsion force by the waves. Reference characters z, z′, and z″ denote vertical displacement, speed, and acceleration of the float, respectively. If the waves are regular waves, the aforementioned formula 5 can be expressed by the following formula:(C−ω2(M+m))z+(N+d)z′=Fz  [Formula 6]
Here, reference character ω denotes a circular frequency of the regular waves. Moreover, the energy of the waves absorbed by the float is calculated on a time average, of (dz′)×z′. Therefore, by making adjustment so that a value in the parenthesis of the left first term of the aforementioned formula 6 becomes zero, the vertical movement of the float can be made to resonate with the wave period, and by adjusting the load attenuation d of the power generator to an appropriate numerical value, power generation efficiency can be improved.
However, the aforementioned wave power generation device 1X has some problems. First, it has a problem that improvement of power generation efficiency of the wave power generation device 1X is difficult. This is because the wave period. Ts changes in accordance with a season or a time zone while the natural period T of the float 4 is determined at the time of design and manufacture.
To solve this problem, it is also conceivable to adjust the natural period T of the float 4 by an adjustment mechanism for adjusting a mass or a restoring force of the float 4. Specifically, such an adjustment mechanism can be considered that a mass is changed by putting ballast water into the float 4 or an added mass is changed by installing a thin plate or the like in a submerged portion of the float 4. Moreover, such an adjustment mechanism can be considered which changes a water-plane area of the float 4 or installs a spring or the like between the float 4 and the body 3.
However, if the aforementioned adjustment mechanism is employed, a second problem occurs that a manufacturing cost of the wave power generation device rises. This is because the cost increases by addition of the new adjustment mechanism. Moreover, along with installation of equipment constituting the adjustment mechanism and the like, a weight of the float increases, its strength calculation or the like becomes necessary, and its design becomes complicated.
Thirdly, employment of the adjustment mechanism also leads to a problem that a failure or a maintenance frequency increases. This is because a possibility that devices constituting the adjustment mechanism and the like causes a failure and the like rises. Particularly, if the wave per generation device 1X is installed in the ocean where an environment is severe, a maintenance frequency further increases.